Generally, the service life of membrane filters is directly dependent on their dirt holding capacity. This is also known as total throughput, the maximum volume quantity Vmax of a liquid solution filtered before the filter is blocked by any particulate contained in the solution. The blocking effect is often based on particulate deposition on the non-filtrate surface of the filter material. Filtration is understood to mean a method for separating solid particles or molecules from a fluid (i.e. liquids or gases), even insoluble liquid droplets from another liquid (emulsion) or from gases (aerosols). A common, essential feature of filtration is that a porous medium, such as a filter paper or membrane, is perfused by the continuous phase (i.e. a liquid or gas), with the solid particles, molecules, or droplets being retained (retention) at the same time on the surface of the porous medium or inside.
Porous membranes are used mainly in the methods of ultrafiltration, of microfiltration, and of dialysis. Whether a particle or molecule is retained by ultrafiltration membranes or microfiltration membranes depends, in addition to the operating conditions, in particular on its size and structure relative to the size and structure of the membrane pores. A typical area of use of microfiltration is, for example, the concentration of suspensions, whereas ultrafiltration is often used for fractionating dissolved low-molecular-weight materials and macromolecules. A complete separation with ultrafiltration requires in this context that the molecular weights of the materials to be fractionated differ by at least one order of magnitude.
The pore size of microfiltration (micro-porous) membranes is in the micrometer range, typically from about 0.08 to about 10 μm. The pore size of ultrafiltration membranes is mostly defined by specifying the limit at which 90% (or 95%) of the molecules of a particular molar mass are retained (molecular weight cutoff, MWCO).
The above-mentioned surface blocking behavior, which may be also referred to as clogging, is specifically present in the field of micro-porous membranes which are commonly applied to sterilizing processes in food and beverage filtration of water, wine, beer or biopharmaceutical filtration of cell and bacteria nutrition media or clarification and purification filtration of cell and bacteria broths. In the afore-mentioned processes, the particulates present in the feed to be filtered show typically a broad distribution, commonly in terms of a Gaussian distribution. Many of the filter devices used in these applications contain two different layers of membrane material. The first layer to be passed by the non-filtrate (feed) is designed for high total throughput, i.e. to retain particulate without being blocked by those as far as possible. The second layer, which is most often a layer with a smaller pore size, is designed to have a sterilizing effect to fully retain small contaminants to be removed from the filtrate, such as microorganisms like bacteria. The enhancement of total throughput and flow rate is primarily targeted to the first layer, less to the sterilizing layer in order to keep this free of defects for the retention of e.g. microorganisms.
The easiest way to increase the total throughput is to enlarge the filter area as such, i.e. the three-dimensional size of the filter material. The enlargement of the filter area is accompanied with higher costs due to an increased amount of the filter material and larger filter device sizes. As a compromise, the filter device size can be kept constant by maintaining its outer dimensions. In such a case, the higher quantity of filter material has to be arranged more compacted in the same device size. The higher compaction results in higher hydrodynamic resistance or vice versa lower flow rates at given pressure difference between the non-filtrate side and the filtrate side of the filter.
In order to increase the flow performance and total throughput of filter materials, several attempts have been proposed in the last decades. For example, DE 10 2011 117 900 A1 discloses a pleated filter element to be used for the filtration of e.g. oil-based suspensions, dispersions or emulsions, comprising a pre-filter layer and a main filter layer. The pre-filter layer comprises recesses which at least partially penetrate into the filter or completely penetrate through the filter layer. The recesses, which may be arranged in form of a pattern, increase the effective filter area and thus the dirt holding capacity of the filter element.
U.S. Pat. No. 6,203,741 B1 and U.S. Pat. No. 6,132,845 A describe methods for forming micro spike thermoplastic liners to be used for sealing tunnels, excavation sites, landfills, i.e. being liquid-impermeable, having at least one roughened surface with a plurality of irregularly shaped projections extending therefrom, which are preferably arranged in a regular pattern, at equal intervals to define columns and rows. The combination of the projections and the roughened surface allows the liner to frictionally engage a desired location. The liners are formed by a calendering process in which a smooth thermoplastic sheet is fed into a calender, which causes the smooth sheet to be formed as a thermoplastic liner having projections extending from one surface thereof.
CA 2 397 022 A1 describes a flat permeable membrane which may consist of polyether sulfones, having recesses on at least one side, wherein the dimensions of the recesses exceed the nominal pore size of the membrane by at least five-fold. The recesses, which may be in form of channels, have an average diameter of 5 to 500 μm, whereas the nominal pore size of the membrane is in the range from 0.2 nm to 5 μm. The thickness of the membrane is described to be from 1 μm to 1000 μm. Said membranes are produced by preparing a substrate, such as a silicon wafer, which has protrusions on its surface as a negative for the desired recesses, applying the membrane material or a precursor thereof onto the substrate and forming the porous membrane on the substrate using solvent evaporation and/or replacing the solvent with a precipitating agent.
U.S. Pat. No. 2006/0016685 A1 discloses textured ion exchange membranes for use in an electrochemical cell, said membranes comprising an anion exchange layer abutting a cation exchange layer to form a heterogeneous water-splitting interface there-between, and a textured surface having a pattern of texture features comprising spaced apart peaks and valleys, wherein the peak to peak distance (dpp) is at least 10 μm and the peak to valley distance (dpv) is at least 10 μm, whereas the aspect ratio dpv/dpp is 0.1 or more.
EP 2 366 449 A2 discloses a polymer membrane having repetitive convex-concave patterns formed on a surface in contact with a fluid to be treated. The membrane which may be made of a polysulfone-based material, has improved permeability and fouling properties, particularly when having a surface roughness of 1.1 to 1.5. The pattern of the membrane may be made using soft lithography technique used to form patterns in the technical field of semiconductors.
U.S. Pat. No. 7,309,385 B2 discloses a gas separation membrane of two or more layers comprising a supporting layer and an organic, porous, gas-permeable separating layer which may be made of polysulfone. The separating layer has a high effective separation area formed in terms of a three-dimensional nanostructure, which may be in form of protruded portions in tube form having a length of several tens nanometers to several millimeters, whereas the thickness of the protruded portion is several nanometers to hundred nanometers.
Further, in U.S. Pat. No. 3,724,673, a thin textured gas-permeable membrane for use in blood oxygenators and dialyzers is described, which comprises a film comprised of thermoplastic material having a myriad of thermoplastically formed deformations or undulations in terms of cones. These cones are plastically deformed in the membrane surface by localized bending and stretching in which case the cone regions are actually thinner than the parent membrane on which they are formed. The deformations or fine undulations constituting the texture are formed by placing a smooth membrane over a die on which a field of cones stands out in relief and applying a vacuum between the die and membrane, so that air pressure deforms the latter in correspondence with the cone pattern.
Further, DE 10 2008 045 621 A1 discloses a gas-permeable and liquid-impermeable membrane used, for instance, for gassing or for gas exchange in blood, wherein the membrane is structured on at least one side, particularly on the non-filtrate side, which may consist of polyether sulfones. The membrane comprises channels and/or branched pathways which may either be in form of through-passages throughout the entire membrane or partially penetrate into the membrane such as blind branches. The walls of the channels have a spacing of 150 μm or less, and the proportion of the membrane surface area which comprises channels and/or branching structures having this spacing constitutes at least 50% of the total surface area of the membrane.
However, most of the aforementioned techniques for improving flow performance and total throughput of filter materials merely rely on increasing the applied area of filter material or have the drawback that the throughput is increased by incorporating channels penetrating through the entire membrane, whereby a desired filtration effect of retaining small contaminants to be removed from the filtrate cannot be achieved.